The invention relates to a spindle motor particularly for the purpose of driving the platters in hard disk drives.
These kinds of spindle motors for hard disk drives comprise a baseplate or a base flange, a stator arrangement that is stationary with respect to the baseplate, a shaft and a bearing system to rotatably support a rotary driven rotor connected to the shaft. The bearing system preferably takes the form of a hydrodynamic bearing system. However, preloaded roller bearing systems can also be used.
The rotor is generally connected to the shaft by means of an interference fit. For this purpose, the rotor is provided with a bore concentric to its rotational axis and having a specific diameter and a specific length into which one end of the shaft is pressfitted. The outside diameter of the shaft is slightly larger than the inside diameter of the rotor bore. This interference is chosen in such a way that a minimum adhesion coefficient, i.e. a specific holding force, is ensured along the entire length of the joint but that a maximum adhesion coefficient is not exceeded which could otherwise cause excessive stress to the components.
The trend for hard disk drives is towards increasingly smaller and flatter disk drives so that the overall height of the spindle motors applied here has to be reduced more and more. One way of reducing the overall height is to reduce the potential length of the joint between the rotor and the shaft with the result that the required holding force may not necessarily be achieved. If the interference is increased in compensation, this can lead to damage to the components during the pressfitting process.
Moreover, due to component and assembly tolerances and an inadequate joint length, greater or smaller deviations between the actual and the theoretic time-invariant rotational axis could occur when the rotor is mounted on the shaft. These deviations can stem from the fact that the central axis of the rotor does not correspond exactly to the central axis of the shaft that carries the rotor. This results in an undesirable large rotor runout (RRO: repeatable runout), both axial (lateral runout) and radial (radial runout), tilting of the rotor with respect to the rotational axis of the shaft and in reduced shock resistance.